The 5E rules around Dash and Disengage are a lot more forgiving and flexible than the equivlents in D&D 3.5. And players used to the older rules may be surprised when the newer rules come into play. Let's dig in.
Opportunity Attacks: In 5E, these are provoked only when an enemy leaves your reach. Approaching you, moving around you, that's fine. Running away, that provokes an OA. It takes a reaction to make an OA, you only get one reaction a round, and you may find better things to use your reaction for, like casting Shield (as a wizard) or deflecting attacks (as a monk). So, one way to avoid OAs is to "flood the zone" — if all 12 kobolds run away at once, most will escape without OAs!
Also note that you can take a reaction for an OA only if you can see the creature provoking one. "You can make an Opportunity Attack when a creature THAT YOU CAN SEE leaves your reach." This opens up retreat options for clever players who can blind their foes. Another effective way to avoid OAs — the target of a successful Shocking Grasp loses the ability to take opportunity attacks, so you can shock and run as well.
Note that with the simplification of rules in 5E, all the things that provoke Attacks of Opportunity in 3.5E no longer do so, from spellcasting to standing up to reloading a crossbow.
You can give yourself immunity to all OAs provoked by movement (which is just about all of them) by using the Disengage action. Note that the Disengage action does not in itself include any movement. If you take Disengage as an action, then you can use your move without provoking OAs. "Your movement does not provoke OAs for the rest of your turn." Disengage does give you immunity to ALL OAs, not, unlike 3.5E, just the ones from your first five feet of movement, using the 3.5 Withdraw action. You can dance your way through a whole crowd without fear.
It is worth noting, however, that the 2024 Sentinel feat lets you take a whack at someone who is Disengaging, and if you hit, they stop moving. But talking a feat that is only useful when the game master makes foes Disengage is asking for a low-use feat. And you do not get too many feats in 5E.
Disengage: For most combatants, Disengage is not much of a battle-ender. You Disengage as an action, so you are giving up your attack. Then you move 30 ft, and you're done. Your enemy can easily move 30 feet and then attack you. It becomes much more of a thing when you can Disengage as a bonus action. That let's you move and Dash, and suddenly you are out of easy melee range. If you do not have Disengage as a bonus action, then you really only have one good option for successfully running away. It is this: Take the Opportunity Attack and move and Dash. If you survive the hit, you are now two moves away, and you keep running.
Note that Disengage does not itself include any movement. A human can Disengage as their action, then move 30 feet using their speed to do so. That's it.
Moving Through: So if Disengage lets you flee foes freely, how do you keep enemies from escaping? Well, one way is to physically block the way. What about moving through an enemy-occupied square? Can you just do that? Short answer, nope. Longer answer, yes, sometimes. "During your move, you can pass through the space of an ally, a creature that has the Incapacitated condition, a Tiny creature, or a creature that is two sizes larger or smaller than you." In the rare times that you can walk through an occupied space, that square is difficult terrain, and costs twice as much move to traverse. But the game lacks an overrun option, so you cannot just muscle your way through an enemy square (aside from the things mentioned before).
What all that boils down to is that simply standing in a bottleneck, such as a doorway, usually suffices to cut off retreat. Bottom line, you can indeed withdraw easily with Disengage, so long as you have a clear path through which to run away. But you can't just run through enemies; you have to run around and between them. But unless the geography of the encounter is in your favor, as an adventurer, you usually don't have the numbers to surround a foe to keep it from escaping.
Pursuit: So let's move on to the chase scenario. When is a fleeing foe gone, and when can you chase it down? The number one most relevant answer comes down to bonus actions. If you have no relevant bonus action, then you are limited to a move with your speed and a move with the Dash action, in one round moving twice your speed (for a human, 60 ft.). In 3.5 there are a full-round run option, but that's gone in 5E. On the other hand, if you can Dash as a bonus action, you get your move, your Dash action, and your Dash bonus action. That's moving thrice your speed in one round (for a human, 90 ft.).
So, how do you get an extra Dash as a bonus action?
- A Rogue's Cunning Action gives it to you at level 2, unlimited.
- The Monk ability called "Monk's Focus" gives you Dash as a bonus action with the Step of the Wind option, also at level 2, also unlimited.
- A small Beastmaster Ranger riding her Primal Companion can use her bonus action to tell her companion to use its bonus action to Dash. That's at level 7, unlimited.
- A Path of the Wild Heart Barbarian can use the Rage of the Wilds Eagle option to Dash with a bonus action — level 3, while raging.
- An Orc's Adrenaline Rush lets you Dash as a bonus action a limited number of times, starting at level 1.
- A Sorcerer, Warlock, or Wizard can cast a first level spell, Expeditious Retreat, which gives you an extra Dash as part of the bonus action casting time and as a bonus action each round for the 10 minute duration. So that's limited too. (Thanks to my good friend Hugh for reminding me of spells!)
- Sorcerers and Wizards can also cast Haste to get Dashes as bonus actions. Several magic items also use Haste.
- A Fighter can Action Surge a second Dash, though that's not bonus action. Very limited uses, level 2.
If you really want to push it, an Eldritch Knight can use Action Surge for an extra Dash, and cast Expeditious Retreat for a bonus action Dash, moving four times move in a single round. That's only for a sprint, though, since both of those have a limited number of uses.
It is worth mentioning that anything that boosts your speed is multiplied by every Dash. A Monk's or Ranger's boost to speed, a Longstrider spell, a Wild Shape that has a great move — all these are multiplied by the number of Dashes you can make.
In an online play-by-post game, I am currently playing a small human Beastmaster Ranger riding a Primal Companion -- a goat, wearing Horseshoes of Speed. That's a base move of 40+30=70, and with a move, a Dash, and a bonus action Dash, Milo and Chocolate are zooming around the battlefield at up to 210 feet per round. That's 24 mph! And they can keep that up indefinitely. So don't even think about running away from Milo!
